1. Field of the Invention
The present invention relates to a voltage controlled oscillator (VCO), and more particularly, to an improved voltage controlled oscillator capable of generating a high frequency with regard to a semiconductor chip operated at a low voltage.
2. Description of the Prior Art
A conventional VCO, as shown in FIG. 1, includes a PMOS transistor 10 with its source connected to a supply voltage VDD and with its gate connected to an input voltage Vin. The conventional VCO also includes a ring oscillator 11 having a plurality of CMOS inverters connected in parallel with each other between the drain of the PMOS transistor 10 and a ground voltage Vss. An inverter 12 inverts the output value from the ring oscillator 11.
The plurality of CMOS inverters in the ring oscillator 11 each respectively includes a PMOS transistor and an NMOS transistor connected in series with each other between the PMOS transistor 10 and the ground voltage Vss. In addition, the output of the last inverter is fed back to the input of the first inverter.
The operation of the conventional VCO will now be described. When the input voltage Vin shifts from 0V to 5V, the PMOS transistor 10 is turned off, thus blocking the supply voltage VDD so that it is not applied to the ring oscillator 11. This results in a change of the equivalent resistance of the plurality of inverters in the ring oscillator 11 and a free running frequency of the inverters. Therefore, a frequency responding to the input voltage Vin can be obtained.
However, in the conventional VCO, the PMOS transistors in the ring oscillator 11 are connected in series with the drain of the PMOS transistor 10 so that it is impossible to operate the VCO at a voltage less than 3V due to the threshold voltage value Vt of the PMOS transistors.
Further, because the PMOS transistor 10 initially lowers the supply voltage VDD, a high frequency cannot be obtained at a chip voltage with regard to the inverters forming the ring oscillator 11.